(a) Field of the Invention
The invention relates to an image display having pixels, the light emitting brightness of which is controlled by display signals. More specifically, the invention relates to an active matrix image display capable of controlling the current supplied to the light emitting element using active elements including an insulated gate FET (field-effect transistor) installed in each pixel.
(b) Description of the Related Art
Generally, a plurality of pixels are arranged in a matrix pattern and images are displayed by controlling the intensity of the light of each pixel based on given display signals in an active matrix image display.
Organic EL image displays are self-luminous displays which have light emitting elements, such as OLEDs (organic light emitting diodes), in each pixel. Organic EL image displays exhibit high visibility of images and a high response speed without requiring any backlight. The brightness of each light-emitting element is controlled by the amount of current supplied to the light emitting element. Namely, the organic EL image display is different from the LCD (liquid crystal displays) in that the light-emitting element is of a current-driven or current-controlled type.
The organic EL image display uses either a simple matrix type driving method or an active matrix type driving method. The simple matrix type driving method is simple in structure but is difficult for realizing a large-size display device and high resolution. Thus, there is increased demand for the earnest development of active matrix methods. In the active matrix type driving method, the current flowing to the light-emitting element in each pixel is controlled by an active element (usually a TFT (Thin Film Transistor) which is a type of an insulated gate FET) provided in the pixel.
In a conventional organic EL image display configured as above, a display operation is performed by a driving method where fixed gray levels (i.e., levels which do not depend on the brightness distribution by input RGB image data) are used for the display operation. That is, the display operation is performed with fixed gray levels, not gray levels which depend on whether the brightness of the display screen determined by brightness distribution of RGB (red, green, and blue) data is high or low. According to the above driving method, however, the brightness difference in a display screen becomes large when a difference between the number of ON-pixels and OFF-pixels is large. The brightness difference generates an uneven display in a screen.
To solve the above-noted problem, Korean publication application No. 2001-14600 (published on Feb. 26, 2001) discloses an active EL display. FIG. 1 shows a reference voltage generation circuit of the active EL display disclosed in Korean publication application No. 2001-14600.
The conventional image display detects the current fed back by a display panel, and generates the reference voltage to be applied to the display panel according to the detected current values. Referring to FIG. 1, the reference voltage generation circuit comprises a cathode end 1 of the display panel, a current detector 2 for converting the current flowing to the cathode end 1 into a voltage, an inverting amplifier 3 for inverting and amplifying an output voltage of the current detector 2, and a current amplifier 4 for amplifying the current of the output signal of the inverting amplifier 3 and generating a reference voltage Vdd to be supplied to EL elements installed in the respective pixels on the display panel.
The conventional image display receives the fed-back current from a common cathode of the EL elements provided to each pixel of the display panel, determines the reference voltage Vdd for supply to the respective EL elements according to the current values, and outputs the determined value. Therefore, the brightness of the EL elements can be controlled through control of the reference voltage Vdd. However, when the reference voltage Vdd is reduced, a number of the gray levels is reduced in the above image display, and when the reference voltage Vdd is supplied to the respective EL elements of the display to cause the current flowing to the EL elements to be varied, the reference voltage Vdd also instantly varies and flickers a display screen.